Physiologically responsive blood pump for ischemia detection and treatment

ABSTRACT

A ventricular assist device incorporating a rotary pump configured to be in fluid communication with a heart and systemic circulation of a subject to assist blood flow from the heart to the systemic circulation. The device includes a pump drive circuit for applying power to the pump, one or more sensors for sensing one or more electrogram signals (such as subcutaneous pre-cordial electrode signals) in the patient, and a signal processing circuit to determine the presence or absence of a reduction in cardiac blood flow, ischemic condition or myocardial infarction condition based on subcutaneous pre-cordial electrode signals, to control power supplied to the pump from the pump drive circuit, and to operate the pump in either a normal sinus rhythm mode in the absence of an ischemic condition or myocardial infarction condition, or a modified mode of operation in the presence of an ischemic condition or myocardial infarction condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 62/245,637 filed Oct. 23, 2015, thedisclosure of which is hereby incorporated herein by reference.

This application fully incorporates by reference U.S. Provisional PatentApplication No. 62/119,895, entitled “Blood Pump for Treatment ofBradycardia,” and U.S. Provisional Patent Application No. 62/232,601,entitled “Blood Pump for Ischemia Detection and Treatment.”

FIELD OF THE INVENTION

The present invention relates to ventricular assist devices (VADs).

BACKGROUND OF THE INVENTION

A VAD is a device which is used to assist the heart of a mammaliansubject such as a human patient. A typical VAD includes a pump which isimplanted in the body of the subject. The pump typically has an inletconnected to a source of blood to be circulated, and an outlet connectedto an artery. Most typically, the inlet of the pump is connected to theinterior of the left ventricle and the outlet of the pump is connectedto the aorta, so that the pump operates in parallel with the leftventricle to impel blood into the aorta. The pump may be a miniaturerotary impeller pump having an impeller disposed in a pump housing anddriven in rotation by a small electric motor which may be closelyintegrated with the pump. The motor in turn typically is powered by animplantable power source such as a storage battery with an arrangementfor charging the battery from an external power source. The VAD may alsoinclude a control system which controls operation of the power source soas to drive the impeller at a set rotational speed and thus provideconstant pumping action.

VADs can be used to assist the heart of subjects suffering fromconditions which impair the pumping ability of the heart. Suchassistance can be provided permanently, or while the subject awaits asuitable heart transplant. In other cases, the assistance provided bythe VAD allows the heart to heal.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present technology provides a ventricular assistdevice or VAD. The VAD desirably may include a rotary pump such as arotary impeller pump implantable in fluid communication with a ventricleand an artery of a subject to assist blood flow from the ventricle tothe artery. The VAD most preferably may further include a pump drivecircuit and also preferably includes one or more sensors for sensing oneor more electrophysiological signals such as subcutaneous ECG signals inthe subject and providing sensor signals representing theelectrophysiological signals. The VAD may also include a signalprocessing circuit connected to the sensors and the pump drive circuit,the signal processing circuit being operative to detect the sensorsignals, and control power supplied to the pump from the pump drivecircuit so that the pump runs in a normal sinus rhythm mode with avarying speed synchronized with the cardiac cycle of the subject. Asfurther discussed below, operation in the normal sinus rhythm modeprovides improved assistance to the heart.

The signal processing circuit may also be operative to determine thepresence or absence of a reduction in cardiac blood flow such asischemia or angina, or total blockage of blood to the heart muscle as inmyocardial infarction, based on the physiological sensor signals and tocontrol power supplied to the pump from the pump drive circuit so as tooperate the pump in a normal sinus rhythm mode in the absence of areduction in cardiac blood flow and to operate the pump in a modifiedmode of operation in the presence of a reduction in cardiac blood flow.For example the modified mode may be a pulsatile mode with differentoperational parameters such as a higher speed as compared to the normalsinus rhythm mode. Or the pump may operate in a non-pulsatile and atconstant speed in the event of detecting a more severe ischemia ormyocardial infarction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a VAD in accordance with oneembodiment of the invention.

FIG. 2 is a schematic diagram depicting a portion of the VAD of FIG. 1.

FIG. 3 is a flowchart depicting a portion of an algorithm used inoperation of the VAD of FIGS. 1 and 2.

FIGS. 4A and 4B are graphs of certain signals and variables occurring inoperation of the VAD of FIGS. 1-3.

FIG. 5 is a diagram depicting the lead and electrode implantation forthe cardiac monitoring of reduced cardiac blood flow.

FIG. 6 is a flowchart of another embodiment depicting a portion of analgorithm used in operation of the VAD of FIGS. 1 and 2.

DETAILED DESCRIPTION

A VAD according to one embodiment of the present disclosure is shown inFIG. 1. The VAD may include an implantable rotary pump 2, incorporatinga motor 4. As used in this disclosure, the term “rotary pump” refers toa pump which incorporates a pumping element mounted for rotation in ahousing. Most typically, the pump 2 is a rotary impeller pump having animpeller mounted within the housing, so that the spinning motion of theimpeller transfers momentum to the fluid to be pumped. Although the pump2 and motor 4 are depicted as separate components for clarity ofillustration in FIG. 1, in practice these components can be closelyintegrated with one another. For example, the impeller of the pump 2 mayserve as the rotor of the motor 4. Most typically, the motor 4 is amulti-phase brushless direct current, permanent magnet motor arranged todrive the impeller of the pump 2 at a rotational speed prescribed by themotor driver by means of a motor commutation technique such astrapezoidal commutation. These components are arranged so that the pump2 can be implanted within the body of a mammalian subject such as ahuman patient, with the inlet 3 in fluid communication with a ventricleof the heart, most typically the left ventricle, and with the outlet 5in fluid communication with an artery, most typically the aorta. Forexample, the pump 2 may be arranged for implantation outside of theheart, and the inlet and outlet may include conduits that can besurgically connected to the ventricle and the aorta. In otherarrangements, the pump 2 is arranged so that it may be implanted withinthe aorta and ventricle. Exemplary implantable pumps are described indetail in U.S. Pat. Nos. 6,264,635, 6,234,772 and 7,699,586; and U.S.Patent Publication No. 20090112312. These patents and published patentapplications, which are commonly assigned, are hereby incorporated byreference.

The VAD may also include a pump drive circuit 6. The pump drive circuit6 may include an electrical storage battery and a motor driver tocontrol the motor. The output of the motor driver may be connected by anoutput connection, such as a cable 9 to the motor 4 of pump 2, so thatthe motor driver can drive the motor 4 and thus operate the pump 2. Themotor driver may typically include semiconductor switching elementswhich are responsive to control signals applied at a control input 7, sothat the current supplied to motor 4 can be controlled. In theparticular arrangement depicted, pump drive circuit 6 may be mountableoutside of the patient's body B and may be connected to the motor 4 byconductors which penetrate the skin of the patient. In otherarrangements, the pump drive circuit 6 may be implanted within the bodyand may be connected to an external power source by an inductivecoupling or skin-penetrating conductors.

Pump 2 optionally may be equipped with a condition sensor 8 such as aspeed sensor. For example, the condition sensor may include a back EMFdetector operative to detect voltage or current in the stator coils ofmotor 4 as a measure of motor speed or load.

The VAD may also include a signal processing circuit 23. The signalprocessing circuit 23 may include an implantable internal module 12 andan external module 18 arranged for mounting outside of the subject'sbody B. Modules 18 and 6 may also be implanted with the patient's body.The signal processing circuit 23 may be connected to the control input 7of pump drive circuit 6. In this embodiment, modules 12 and 18 areconnected to one another by a suitable signal transmitting arrangementsuch as radio frequency telemetry transmitting and receiving units 16 sothat signals and data can be interchanged between the modules. Modules12 and 18 may include conventional data processing elements such as oneor more microprocessors 15 and one or more memory elements 13 arrangedto perform the algorithms discussed below. The distribution of hardwareelements and software functions between these modules can be varied overa wide range. At one extreme, all of the data processing necessary toperform the algorithm is performed in external control module 18, andthe internal module acts essentially as a conduit for data and signals.At the other extreme, all of the hardware and software required toperform the algorithms resides in the internal module 12, and theexternal module is omitted. The power required to operate the electroniccircuitry of the internal module 12 typically is about 3 orders ofmagnitude less than the power required to drive motor 4. The internalmodule 12 may be connected to receive power from the alternating currentsupplied by the pump drive circuit 6 to motor 4. This arrangement isparticularly useful where the internal module 12 is physically locatedat the pump 2. Where the internal module of the signal processingcircuit 23 is physically located at the pump 2, it may be desirable toprovide magnetic shielding between the coils of the pump motor 4 and thecircuitry of the internal module 12. Alternatively, where the internalmodule 12 is located away from the pump 2, then the signal processingcircuitry 23 can receive power from an internal battery such as aprimary battery or rechargeable battery.

The VAD further includes sensors 10 which may be connected to theinternal module 12 of the signal processing circuit 23. As shown ingreater detail in FIGS. 2 and 5, the sensors include subcutaneouselectrodes implanted at pre-cordial (or chest) locations similar to a 12lead ECG cardiac monitor such as designations V1 through V5. Recordingsfrom these electrodes could be of unipolar configuration with a farfield remote anode 41 (FIG. 2), such as the conductive titanium case ofthe implanted electronics 22 connected to receive electrical signalsfrom the subcutaneous pre-cordial electrodes 30, 32, 34, 36. Theelectrical recordings could also be bipolar with respect pairs ofelectrodes. When the VAD is installed, the subcutaneous pre-cordialelectrodes 30, 32, 34, and 36 are disposed at appropriate locations nearthe heart of the subject.

As shown in FIG. 2, the sensors 10 optionally also include one or morephysiological condition sensors 43. The physiological condition sensors43 can be used to sense and transmit any type of physiologicalparameter, including but not limited to oxygen concentration, pressurewithin a vessel or chamber, and temperature. Sensors 10 optionally mayalso include one or more further sensors 45 arranged to provide a signalrepresenting a parameter related to cardiac demand. For example, thefurther sensors 45 may include one or more accelerometers arranged toprovide signals representing movement of the patient's body B. There maybe a positive correlation between the amount of movement and cardiacdemand.

The various sensors are connected to the internal module 12 of thesignal processing circuit 23 through appropriate signal conditioningelements such as an analog to digital converter 47 and buffer memory 49.

Signal processing circuit 23 may be configured with functionality toreceive, analyze and process signals received from sensors as theyrelate to the physiological condition of the patient. Some of thisfunctionality may include processing signals from sensors 10 todetermine the phase of the patient's cardiac cycle; sensing thepatient's intrinsic heart rate; determining the patient's metabolicdemand; and detecting a reduction of blood flow to the heart duringconditions of ischemia or during more significant reduction as withmyocardial infarction, and in response to those signals, may set themode of operation and speed of the pump 2 accordingly. The signalprocessing circuit 23 may also control the frequency of the motor drivesignal to the pump 2.

Signal processing circuit 23 may be specifically configured torepeatedly execute an algorithm as shown in simplified form in FIG. 3.At step 102, a processor 15 may execute a beat detection routine usingsignals acquired from the subcutaneous electrodes 30, 32, 34 and 36.Beat detection algorithms based on pre-cordial electrode signals arewell known in the art and are commonly employed in devices such as 12lead ECG cardiac monitors. Any detection circuit or algorithm routinewhich is effective to discriminate a normal sinus beat versus an ectopic(non-sinus) beat can be employed.

At step 104, the algorithm branches depending on the results of theindividual beat detection. If the detection has determined by processor15 that the subject's heart is a non-sinus (or an ectopic beat), thealgorithm ignores the beat 112 and returns to the beat detectionalgorithm 102. If the beat determination algorithm 104 as determined bythe processor 15 that the beat is of normal sinus origin, then thealgorithm moves on to the ST segment measurement function 106 whichwould be performed by processor 15. Once the ST segment amplitude ismeasured 106 by the processor 15, the algorithm determines whether thereis a ST segment level deviation sufficiently greater than a specifiedamplitude (positive and/or negative) 116. This ST segment measurement byprocessor 15 may also be determined by calculating a moving average ofmultiple detected normal sinus beats. The moving average data may bestored in memory 13. If the ST segment measurement exceeds a specifiedthreshold for an ischemic condition, then the algorithm determines thatan ischemic reduction of blood flow to the cardiac muscle has occurred,the VAD pump drive 6 is instructed to operate in an ischemia mode 114 asprescribed (e.g., programmed) by the physician and controlled byprocessor module 18. For example the ischemia operational conditions maybe an increase in the pulsatile speed while still synchronized to thenatural heart beat. Depending upon the magnitude and polarity of the STsegment deviation, the algorithm may determine that the cardiac muscleis in a myocardial infarction condition. If the ST segment measurementexceeds a threshold indicative of a myocardial infarction, the VAD pumpdrive 6 is instructed by the processor module 18 to operate in anmyocardial infarction mode 119 as prescribed (e.g., programmed) by thephysician. For this condition, the myocardial infarction mode may be toswitch the pump drive 6 to a constant speed mode in order to increasethe cardiac output.

If there is no ischemia or myocardial infarction detected, then the VADmay continue to operate in the normal sinus rhythm mode 118. In thismode, the signal processing circuit 23 actuates pump drive circuit 6 tovary the speed of the pump 2 between a minimum speed and a maximumspeed, as depicted by curve 108 (FIG. 4B). The pattern of variation inthe speed of the pump 2 is synchronized with the intrinsic rhythm of thepatient's heart as shown by the subcutaneous ECG signals so that thevariation in speed of the pump 2 has a substantially fixed phaserelationship to the intrinsic rhythm of the heart. Most preferably, thepump 2 operates at maximum speed during ventricular systole, when theventricles contract to expel blood. The ECG curve shown in FIG. 4A is aschematic depiction showing a conventional external electrocardiogramwaveform, which represents a composite of the electrical signals in theentire heart. In practice, the actual subcutaneous ECG signals appearingon electrodes 30, 32, 34 and 36 (FIG. 5) would be recorded and/ormeasured as separate signals. The recorded data may be stored in memory13 for future analysis against suspected non-sinus beats.

In another embodiment, signal processing circuit 23 repeatedly executesan algorithm as shown in simplified form in FIG. 6. At step 202, aprocessor may execute a beat detection routine using signals acquiredfrom the subcutaneous electrodes 30, 32, 34 and 36. Beat detection basedon pre-cordial electrode signals are well known in the art and arecommonly employed in devices such as 12 lead ECG cardiac monitors. Anydetection circuit or algorithm routine which is effective to detect anormal sinus beat versus an ectopic (non-sinus) beat can be programmedinto the processor 15.

At step 204, the algorithm branches depending on the results of theindividual beat detection. If the detection has determined that thesubject's heart is a non-sinus beat, the algorithm ignores the beat 212and continues with the ventricular tachy-arrhythmia rhythm detectionalgorithm 213. If the ventricular tachy-arrhythmia rhythm detectionalgorithm determines that there is no ventricular tachy-arrhythmiarhythm present, and is a non-sustained condition, then the algorithmreturns to the beat detection 202. If the ventricular tachy-arrhythmiadetection 213 determines that a ventricular tachy arrhythmia is present,the algorithm enters a ventricular tachy-arrhythmia mode 215.

If the beat detection algorithm 204 determines that the beat is ofnormal sinus origin, then the algorithm moves on to the ST segmentmeasurement function 206. Once the ST segment amplitude is measured, thealgorithm determines whether there is a ST segment level deviationsufficiently greater than a specified amplitude (positive and/ornegative) 216 as calculated by the processor 15. This ST segmentmeasurement may also be determined by calculating a moving average ofmultiple detected normal sinus beats. The moving average data may bestored in memory 13. If the ST segment measurement exceeds a specifiedthreshold for an ischemic condition, then the algorithm determines thatan ischemic reduction of blood flow to the cardiac muscle has occurred,the VAD is instructed to operate in an ischemia mode 214 as prescribed(e.g., programmed) by the physician and controlled by processor module18. Depending upon the magnitude and polarity of the ST segmentdeviation, the algorithm may determine that the cardiac muscle is in amyocardial infarction condition. If the ST segment measurement exceeds athreshold indicative of a myocardial infarction, the VAD pump drive 6 isinstructed by the processor module 18 to operate in an myocardialinfarction mode 219 as prescribed (e.g., programmed) by the physician.For this condition the myocardial infarction mode may be to switch thepump drive 6 to a constant speed mode in order to increase the cardiacoutput. If there is no reduction in cardiac blood flow detected, thenthe VAD operates in the normal sinus rhythm mode 218. In this mode, thesignal processing circuit 23 actuates pump drive circuit 6 to vary thespeed of the pump 2 varies between a minimum speed and a maximum speed,as depicted by curve 208 (FIG. 4B). The pattern of variation in thespeed of the pump 2 is synchronized with the intrinsic rhythm of thepatient's heart as shown by the subcutaneous ECG signals so that thevariation in speed of the pump 2 has a substantially fixed phaserelationship to the intrinsic rhythm of the heart. Most preferably, thepump 2 operates at maximum speed during ventricular systole, when theventricles contract to expel blood. The ECG curve shown in FIG. 4A is aschematic depiction showing a conventional external electrocardiogramwaveform, which represents a composite of the electrical signals in theentire heart. In practice, the actual subcutaneous ECG signals appearingon electrodes 30, 32, 34 and 36 (FIG. 5) may be separate signals. Eachelectrode may provide additional electrical vectors (view) of thecardiac contraction as indicated during the QRS of the heart beat.

The therapeutic modes as identified in FIG. 6 may be exemplified withinthe following table:

TABLE 1 VAD Operational Modes Based on Detected Conditions NormalTachycardia Reduced Cardiac Sinus Arrhythmia Blood Flow Modes RhythmModes Myocardial Mode VT VF Ischemia Infarction Continuous Flow ✓Pulsatile - ✓ ✓ during synchronous recovery (co-pulse) Pulsatile - Or ✓✓ at onset synchronous (counter-pulse) Pulsatile - ✓ asynchronousIncrease Modulated ✓ ✓ ✓ ✓ Pump Speeds Decrease Modulated Pump SpeedsIncrease Pulse Duty ✓ Cycle Decrease Pulse Duty Cycle

For example, if ischemia (or reduced blood flow to the cardiac muscle)is detected, the response by the VAD could be an increase in themodulated pump speed to increase the over cardiac output. In comparisonduring a more serious condition of myocardial infarction, the therapymode could change during the detected onset of the infarction andrecovery between co-pulsation and counter-pulsation methods of providingpulsatility. The counter-pulsation method which increases the speedduring diastole reduces the cardiac load variation whereas theco-pulsation method increases the pulse pressure. Depending on thephysiological conditions detected during the infarction, the VAD canalternate between the two methods during the onset and post infarctionrecovery. With respect to ventricular arrhythmia conditions, thedetection of ventricular tachycardia or ventricular fibrillation wouldresult in providing two different therapeutic modes. The therapeuticmode for ventricular fibrillation would be a continuous flow mode withan increase pump speed, since there is no cardiac pulse. In comparisonthe therapeutic mode for ventricular tachycardia could impact the pumpspeed and/or duty cycle, providing an asynchronous pulsatile output.

As ventricular systole occurs during the R-wave of the subcutaneous ECGrepresenting ventricular depolarization, the pump 2 desirably reachesmaximum speed at a time close to the timing of the R-wave. The signalprocessing circuit 23 can use various features of the subcutaneous ECGsignals as the basis for synchronization. An ECG signal of the leftventricle using subcutaneous electrodes 34 or 36 (FIG. 5) provides thetiming of the ventricular depolarization. The signal processing circuit23 can simply actuate pump drive circuit 6 to increase the speed of pump2, each time the left ventricle signal indicates beginning ofventricular depolarization, e.g., at the beginning of the R-wave.However, the mechanical components of pump 2 have inertia and require afinite time to accelerate from minimum speed to maximum speed. This timeis referred to herein as the slew time T_(S) (FIG. 4B). To allow forthis effect, the signal processing circuit 23 may actuate the pump drivecircuit 6 to progressively increase speed of the pump 2 over a periodequal to T_(S).

The signal processing circuit 23 can time the beginning of this periodT_(R) from the R-wave of the preceding cardiac cycle. The cycle timeT_(C) of the cardiac cycle is simply the inverse of the heart rate.Thus, the signal processing circuit 23 can initiate the increase in thepump speed at a time T_(R) after the R-wave of the preceding cycle,where T_(R)=T_(C)−T_(S). Provided that the heart rate is constant orvarying slowly, and that the signal processing circuit 23 updates theheart rate and recalculates T_(C) frequently, this simple arrangementcan yield reasonable synchronization of the pump speed increase with theonset of ventricular systole. The cycle time T_(C) used in thiscalculation can be based on a moving average of the cycle time over afew cycles.

Alternatively or additionally, the signal processing circuit 23 canmeasure the synchronization achieved during each cardiac cycle andadvance or retard the initiation of pump acceleration accordingly. Forexample, if T_(R) was too short in the preceding cycle, so that the pump2 reached full speed before the R-wave, the signal processing circuit 23can increase T_(R) for the next cycle. Thus, the signal processingcircuit 23 can act as a phase-locked loop holding the pump speedwaveform in synchronization with the intrinsic cardiac cycle of thepatient. In this arrangement, the cyclic variation of pump speed has afixed phase relationship to the R-wave. In a variant of thisarrangement, the measurement of synchronization can be a moving averagerepresenting the last few cardiac cycles.

In normal sinus rhythm, there is a substantially constant interval fromthe P-wave to the R-wave in each cardiac cycle. This interval can beestimated from the heart rate or can be determined directly frommeasurement of the subcutaneous ECG signals. Thus, the signal processingcircuit 23 can time a period T_(D) (FIG. 4B) after each P-wave andinitiate pump acceleration at the end of this period. T_(D) may beselected to equal the P-wave to R-wave interval minus T_(S). In someinstances, T_(S) may be equal to the P-wave to R-wave interval, in whichcase T_(D) may be zero. In this arrangement, the cyclic variation ofpump speed has a fixed phase relationship to the P-wave.

Many other features of the subcutaneous ECG can be used as the basis forsynchronization. Software routines for recognizing individual featuresof the waveforms such as the P-wave, and QRS complex of an ECG are knownper se, and any such routine can be used in the synchronization scheme.

Synchronizing the VAD with the patient's intrinsic depolarization willallow the pump 2 to operate when it is most advantageous to do so.Cardiac output is greatest during contraction of the atria andventricles. In a weak or diseased heart, contraction of the chambers,and particularly the left ventricle is when assistance from a VAD ismost critical. Therefore timing of the pump 2 with ventricularcontraction will provide the optimal assistance to the patient andmaximize the therapeutic effect of the VAD. Moreover, operation in apulsatile mode synchronized to the subject's cardiac cycle can improveefficiency and thus conserve power.

While the synchronization of the pump 2 can be triggered by the actualoccurrence of an electrophysiological signal, it is also possible toprogram the signal processing circuit 23 to anticipate the impendingoccurrence of a particular subcutaneous ECG waveform. For example, it iswell known that each phase of the cardiac cycle should last forapproximately the same duration of time in healthy patients. Through aprogrammed algorithm, the processor could be programmed by methods knownin the art to measure and store historical patient data in a memory 13.This memory 13 could be located anywhere within the circuitry of theVAD, or externally.

The data would consist of how long each phase of the cardiac cycle lastsin a given patient in a predetermined time. Measurements taken andstored over time can be used to determine through any mathematic orstatistical means known, when the next phase of the cardiac cycle shouldbegin in a given patient. This method would allow the processor 15 toinstruct the pump drive circuit 6 when to accelerate the pump 2, basedon an anticipated subcutaneous ECG waveform. Because atrial andventricular systole is signaled by the beginning of the P-wave andR-wave respectively, the historical analysis of these phases of thecardiac cycle could be used to predict the onset of systole.

This predictive method of synchronizing the pump 2 with an actual oranticipated subcutaneous ECG waveform is of particular use in patientssuffering from left side heart failure. Left side heart failure is achallenging pathology predominantly affecting the left ventricle.Patients with left side failure require assistance in order to maximizethe efficiency of the left ventricular contraction. In one embodiment ofthe present invention, the signal processing circuit 23 will receivesubcutaneous ECG waveform signal information from a patient with leftsided heart failure. The signal processing circuit 23 will analyze thesignal information and determine when an R-wave is occurring or is aboutto occur. Upon detecting the occurrence or impending occurrence of thecomplex, the signal processing circuit 23 will instruct the motor 4,through the pump drive circuit 6, to operate in order to drive the pump2 in synchronism with the patient's own ventricular systole.

The signal processing circuit 23 can set the duration D_(I) (FIG. 4B) ofpump operation at maximum speed during each cardiac cycle based on thehistorical timing of the patient's R-waves. Alternatively, D_(I) can beset as a fixed proportion of the cardiac cycle time T_(C). In yetanother variant, the signal processing circuit 23 is arranged so thatD_(I), or the routine used to set D_(I), can be selected by thephysician. Typically, D_(I) is selected so that the pump 2 operates atmaximum speed during most or all of ventricular systole.

The maximum speed of the pump 2 or the D_(I) can be fixed values, or canbe set by the signal processing circuit 23 depending on sensed dataindicating the current status of the patient. For example, the maximumspeed may increase with the heart rate as determined by the subcutaneousECG signals from the electrodes, or as determined based on readings fromphysiological condition sensor 43 (FIG. 2), cardiac demand parametersensor 45 or some combination of these. Thus, the maximum speed may varydepending on whether the patient is sleeping, awake, and/or exercising.The minimum speed typically is non-zero speed, so that the pump 2 runscontinually but speeds up and slows down during each cycle. For example,some rotary impeller pumps utilize hydrodynamic bearings to maintain apump rotor out of contact with a surrounding pump housing, so that thepump operates with zero wear on the rotor and housing. Thesehydrodynamic bearings become ineffective when the rotor speed fallsbelow a minimum pump operating speed. When the pump 2 incorporates suchbearings, the minimum speed set by the signal processing circuit 23desirably is set at or above the minimum pump operating speed. Theminimum speed can also vary depending on sensed data.

Curve 108 (FIG. 4B) depicts the speed variation as a progressive ramp-upfrom minimum to maximum, followed by operation at maximum, followed byramp-down to minimum and operation at minimum. However, the pattern ofspeed variation can be more complex, with the speed continuously varyingduring the entire cycle. Here again, however, the pattern of speedvariation is synchronized with the patient's intrinsic cardiac cycle inthe manner described above.

The VAD continues to operate in the normal sinus rhythm mode describedabove while the signal processing circuit 23 continuously executes beatdetection 102 (FIG. 3). So long as the patient remains in normal sinusrhythm, the normal sinus rhythm mode 118 operation continues. However,if an ischemia condition is detected, the program passes to step 114,where the signal processing circuit 23 actuates pump drive circuit 6 tooperate the pump 2 in a mode referred to herein as the ischemia mode112. In one arrangement, the ischemia mode 114 is a constant-speed modein which the pump 2 runs at a constant speed and the pump speed does notvary during the cardiac cycle. In ischemia, constant speed mode, signalprocessing circuit 23 actuates the pump drive circuit 6 to supply powerat a constant frequency to the motor 4 of pump 2, so that the pump 2operates at a constant speed. This speed desirably is less than themaximum speed used during pulsatile operation. While the pump speed issubstantially constant during the cardiac cycle, the signal processingcircuit 23 optionally can alter the constant speed depending onconditions detected by the physiologic sensor 43.

In another arrangement, the ischemia mode 114 or the myocardialinfarction mode 119 may be a pulsatile mode in which variation of thepump speed is synchronized to the sinus beats. The signal processingcircuit 23 may include an algorithm to select either a pulsed mode(synchronous or asynchronous) or constant speed mode in response todetecting an ischemia condition or myocardial infarction condition anddepending on conditions such as metabolic demand.

While the VAD is in ischemia mode 114, the signal processing circuit 23continually executes the beat detection routine 102 and the ST segmentmeasurement routine 106. If routine 106 detects a return to normal sinusrhythm, indicating that the ischemia condition has passed, the normalsinus rhythm mode 118 is restored as the pump drive 6 is instructed bythe processor module 18. With a long term implant like a VAD, the normalsinus rhythm subcutaneous ECG data would be stored in memory 15 and maybe updated on a continuous basis to reflect subtle changes to thebaseline waveform and rhythm. This ongoing update of the normal sinusrhythm waveform would be performed by processor 13 and the updatedwaveforms stored in memory 15.

In still other embodiments, the ischemia detection and myocardialinfarction detection, and the response discussed above can be applied inVADs having pumps other than rotary pumps.

While various elements have been described above as individualcomponents depicted in functional block diagrams, these elements can becombined with one another. Conversely, elements shown as unitaryelements in the functional block diagrams discussed above can beseparated into separate elements. Also, the features described abovewith reference to different embodiments of the invention can be combinedwith one another.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

The invention claimed is:
 1. A ventricular assist device comprising: arotary pump configured to be implantable in fluid communication with aheart and systemic circulation of a subject to assist blood flow fromthe heart to the systemic circulation; a pump drive circuit for applyingpower to the pump; a pump drive circuit to control the speed of thepump; one or more sensors for sensing one or more electrogram signals ina patient; and a signal processing circuit in communication with thesensors and the pump drive circuit, the signal processing circuit beingoperative to receive the subcutaneous ECG signals, the signal processingcircuit is operative to: determine the presence of cardiac ischemiacondition based on subcutaneous pre-cordial electrode signals, and tocontrol power supplied to the pump from the pump drive circuit so as tocontrol the speed of the pump and operate the pump in a normal sinusrhythm mode in the absence of an ischemic condition, and to operate thepump in an ischemic mode of operation based on the presence of anischemic condition, the ischemic mode of operation including selectivelyactivating a non-pulsatile mode and running the pump in a non-pulsatilemanner different from the normal sinus rhythm mode.
 2. A ventricularassist device as claimed in claim 1, wherein the pump is a rotaryimpeller pump.
 3. A ventricular assist device as claimed in claim 1,wherein the signal processing circuit is operative to adjust the varyingspeed responsive to a condition of the subject.
 4. A ventricular assistdevice as claimed in claim 3, wherein the signal processing circuitadjusts the varying speed in accordance with electrophysiologicalsignals received by the circuit from the sensors.
 5. A ventricularassist device as claimed in claim 1, in which the signal processingcircuit is operative to control power supplied to the pump so that thevarying speed has a substantially fixed phase relationship with theP-wave of the subject's cardiac cycle.
 6. A ventricular assist device asclaimed in claim 1, wherein the electrophysiological signals includesubcutaneous ECG waveforms.
 7. A ventricular assist device as claimed inclaim 6, wherein the subcutaneous ECG waveforms include at least oneunipolar signal.
 8. A ventricular assist device as claimed in claim 1,wherein the signal processing circuit is operative via the motor driverto control power supplied to the pump so as to vary the speed of thepump in the non-pulsatile mode based on a condition of the subject.